Apple has released iOS 8.4 back in June and the main changes were a redesigned Music app with Apple Music support and some new iBooks features. Google would have simply updated Play Music and Play Books and all Android users would have gotten the new features without manually installing a new Android version.
The only way for Apple to add new features to Safari, Apple Maps, App Store, the music app, the calendar app, the mail app, iBooks and all the other built-in apps is to update the operating system. That's an outdated model and it's pretty inefficient.
Everytime you hear about a new release for Google's Android apps (Chrome, Play Store, Play Music, Play Books, Gmail, Google Drive, YouTube and more), that could be a new Android version. Some Android OEMs like Motorola, Sony and HTC moved core apps to the Play Store to update them faster.
It's worth pointing out that iOS updates include many security updates and other bug fixes. Right now, these updates can't be installed without an operating system update in Android, but that may change in the future. Google promised to release monthly security patches for Nexus devices, while Samsung and LG will also update their devices monthly.
Android's OEMs update their devices faster then a few years ago, but the process is still slow because of the carriers, custom user interfaces like TouchWiz and the number of devices released every year.
